This invention relates in general to pulse generators and relates more particularly to a pulse compressor that is capable of generating large amplitude, small width pulses. Generation of picosecond pulses is necessary for many applications including applications involving characterization of very high speed transient phenomena or very high bandwidth sampling, mixing, or speed/delay-time testing. In these applications, the pulses can be used as a trigger, an excitation signal or a time base reference.
Picosecond pulses are presently generated by several different techniques. In Hewlett-Packard Application Note 918 entitled PULSE AND WAVEFORM GENERATION WITH STEP RECOVERY DIODES, a step recovery diode is used to produce a travelling wave pulse having a very steep leading edge. This pulse travels along a transmission line past a point at which an output signal is tapped from the transmission line. This wave is then reflected back with opposite polarity past the output tap so that the resulting output signal is a pulse of width equal to the rise time of the leading edge of the travelling wave pulse plus the round trip time for the pulse to travel from the output tap to the reflection point and back to the output tap. As a result of this, these pulses in general have a Full Width at Half Maximum (FWHM) greater than 35 picoseconds. These pulses are inherently limited by the minority carrier lifetime of the step recovery diode.
In W. C. G. Ortel, THE MONOSTABLE TUNNEL DIODE TRIGGER CIRCUIT, Proc. IEEE, vol. 54, pp. 936-946, July 1966, is presented a pulse generator that utilizes tunnel diodes in place of the step recovery diodes. Unfortunately, although narrower pulses can be achieved than with step recovery diodes, the output voltage is at most a few hundred millivolts.
In M. B. Ketchen, et. al., GENERATION OF SUBPICOSECOND ELECTRICAL PULSES ON COPLANAR TRANSMISSION LINES, Appl. Phys. Lett., vol. 48, pp. 751-753, Mar. 24, 1986, picosecond and subpicosecond pulses are generated by photogeneration of carriers. Unfortunately, this technique requires expensive, sophisticated and bulky lasers to generate picosecond or subpicosecond optical excitation pulses. In addition, bulk optics is needed to focus the laser beam onto the circuits.
In S. M. Faris, GENERATION AND MEASUREMENT OF ULTRASHORT CURRENT PULSES WITH JOSEPHSON DEVICES, Appl. Phys. Lett., vol. 36, pp. 1005-1007, June 15, 1980, picosecond pulses are generated using superconducting Josephson junction technology. Historically, this has required expensive apparatus to maintain the temperatures required for superconductivity. Although it appears that room temperature superconductivity may be possible, at present it is still only a hope for the future.
The Colby Instruments, Inc. model PG 5000A pulse generator is an example of a conventional commercially available pulse generator. This technique uses an active device, such as an FET, to amplify and clip an input signal leaving only the high slew rate portion of the resulting waveform. These pulse generators are expensive and are limited to rise/fall times greater than 35 picoseconds and pulsewidths greater than 100 picoseconds.
In D. Jager, CHARACTERISTICS OF TRAVELLING WAVES ALONG THE NON-LINEAR TRANSMISSION LINES FOR MONOLITHIC INTEGRATED CIRCUITS: A REVIEW, Int. J. Electronics, vol. 58, pp. 649-669, 1985, a distributed pulse compression technique has been proposed, but has not been reduced to practice. In direct analogy to compression of optical pulses, this method utilizes dispersion and nonlinearity in the compressing medium in which the pulse envelope is slowly varying compared to the carrier frequency. This technique is not applicable for direct electrical compression of pulses in which no carrier signal is involved. This technique utilizes a wide band of frequencies in excess of 200 GigaHertz and solution of the associated soliton problem involves the nonlinear Schroedinger equation. In contrast to this, the direct pulse compression technique presented herein involves the band of frequencies from dc to about 100 GigaHertz and the associated soliton problem involves the Kortweg-deVries equation.
In D. H. Auston, M. C. Juss and P. R. Smith, ELECTRICAL PULSE COMPRESSION BY TRAVELLING-WAVE PHOTOCONDUCTIVITY, Picosecond Electronics and Optoelectronics Conference Technical Digest, pp. 188-190, January 1987, a laser pulse produces a local high conductance spot in a transmission line that produces a travelling-wave pulse that acts effectively as a moving short. In theory, the relativistic Doppler effect will compress an incident electrical pulse in the transmission line, but this device has also not been reduced to practice. Also, this technique requires an external laser source, bulk optics and diffraction gratings, thereby increasing cost and complexity and preventing the pulse compressor from being manufactured as a monolithic structure.
It would be advantageous to have a design that enables production as a monolithic structure in order to reduce size, reduce manufacturing costs, reduce the effects of parasitic circuit elements as compared to lumped or hybrid configurations, and improve reliability. Most importantly, a monolithic structure enables the generation of electrical pulses with widths as small as several picoseconds. It would also be advantageous to avoid the expense of external lasers, external optics, and/or cryogenic cooling as are needed in the optical and Josephson junction devices discussed above.
In accordance with the present invention, a monolithic design is presented in which electrical pulses are directly compressed by use of a transmission line having both nonlinearity and dispersion. Fourier analysis of waveforms shows that pulses having a steep leading edge must have Fourier components of period comparable to the pulse rise time. Thus, in order to compress a pulse, an increased amount of high frequency components must be generated by the compressor. This generation is provided by the nonlinearity of this pulse compressor.
Wave propagation on a dispersive, nonlinear transmission, ,line is in general characterized by an initial steepening of the input rising edge (i.e., shock wave formation) followed by a breakup into stable solitary waves, called solitons. The nonlinearity causes the formation of a shock front corresponding to the generation of higher frequency harmonics. Because of dispersion, these harmonics propagate at different velocities which in turn oppose the formation of the shock wave (i.e., broaden the fast rising edge). The dynamic interaction between the nonlinearity and the dispersion proceeds until a balance is achieved whereby the amplitudes of the harmonics stay fixed and their phase velocities are equal, resulting in a wave of permanent profile called a soliton.
In such a transmission line, an electrical pulse injected into an input end of the transmission line, will in general break up into a set of solitons which then travel independently at different velocities along the transmission line. The velocity of each soliton depends on the amplitude of that soliton so that, not only are multiple pulses produced instead of a single pulse, it becomes difficult to predict the temporal interval between the times at which these pulses reach the output of the pulse generator. Such a situation is not well suited for a pulse generator.
In accordance with the illustrated preferred embodiment, a loaded transmission line is presented that has nonlinearity and dispersion selected to compress a pulse without producing a significant amount of unwanted secondary pulses. Loading the transmission line at varying intervals with varactors introduces both nonlinearity and dispersion. The unloaded transmission line should have a low enough amount of dispersion that the dispersion of the loaded transmission line is determined by the locations and parameters of the varactors used to load the transmission line. The pulse that is injected into the input end of the transmission line is selected to have a profile that approximately matches the profile of a stable soliton. As a result of this match, any breakup of this input pulse results in a large primary soliton and a low level oscillatory tail which correspond to energy that is not coupled into the primary soliton. The degree of match is chosen so that the secondary solitons contribute only a negligible amount of signal to the ripple in the output signal of the pulse generator.
The most, important property of solitons for the present compressor is that the pulsewidth of a stable soliton decreases with increasing amplitude, lower dispersion and higher nonlinearity. Therefore, the transmission line is selected to have a ratio of dispersion to nonlinearity that decreases in the direction of propagation. This can be achieved by decreasing the dispersion and/or increasing the nonlinearity along the direction of propagation.
The pulse compressor consists of a loaded transmission line having successive sections in each of which this ratio is substantially constant. The sections are designed so that this ratio decreases from section to section along the direction of pulse transmission. The limiting case of a single varactor per section has the advantage of reduced length for a given total change in the ratio of dispersion to nonlinearity. The values of inductance per section and capacitance per section are each varied in a manner that, for a fixed amount of compression and number of compressor sections, minimizes the fraction of input energy that is not coupled into the primary soliton.
This design is advantageous in that the operation of many types of devices, such as samplers, triggers and pulse generator/driver circuits can be easily enhanced by the monolithic integration of such a pulse compressor with such circuits. For example, extremely narrow laser pulses can be generated by triggering the laser with pulses generated by a pulse generator that incorporates this pulse compressor.